how does using geothermal power plant can cause possible pollution?

a. since vents are not widely available, man cannot just build geothermal power plant
b. geothermal power plants are designed to operate 24 hours a day throughout the whole year
c. processes like drilling on earth's crust may release some poisonous gas and minerals which could pose harm to the environment
d. installation cost of steam power plant is quite high and there is no guarantee that the amount of energy to be harnessed could justify the operation cost​